1. Field of the Invention
The present invention relates to high-speed rotary atomisers for applying powder coating. Such atomisers comprise a housing; a rotatable bell disc arranged at the front of the housing; a motor driving the bell disc accommodated in the housing; at least one powder supply channel passing through the housing and emerging at the front of the housing and a guidance air arrangement with which pressurised guidance air is blowable in the direction of the powder cloud generated by the bell-shaped disc.
2. Background Art
High-speed rotary atomisers have been increasingly used in recent times for the application of powdered coatings. The powder cloud generated by high-speed rotary atomisers needs to be shaped. To shape the powder cloud generated by such an atomiser, guidance air arrangements in the form of a plurality of bore holes arranged at the front of the housing are used. Pressurised guidance air emerges from these bore holes towards the powder cloud and produces the desired shaping effect. A disadvantage of this known guidance air arrangement is that as the guidance air passes through the different bore holes, it forms turbulence which has a detrimental effect on the powder cloud. In addition, the quantity of guidance air which can be supplied to the powder cloud in this way is limited.
An object of the present invention is to configure a high-speed rotary atomiser of the type described above such that the guidance air flows towards the powder cloud as uniformly and as free of turbulence as possible.
In the present invention, the guidance air arrangement includes a rotationally symmetrical air guidance body attached to the front of the housing and a jacket surface to which the guidance air can be so fed that it flows out forwards along the jacket surface as a substantially rotationally symmetrical air curtain, the air guidance body having an axial through-bore through which the coating powder flows towards the bell disc.
According to the invention, therefore, the guidance air is not passed through small bore holes but along an external jacket surface of a more or less solid body, without the guidance air being subjected to any radially outward physical restriction. The guidance air curtain has no interruptions and is therefore not divided into separate streams, which could only be reunited while generating turbulence. Moreover, relatively large quantities of guidance air can be moved over the relatively large jacket surface of the air guidance body, so that the effect of the guidance air on the powder cloud is far greater than with known high-speed rotary atomisers.
An especially preferred embodiment of the invention is that in which a rear, radial surface of the air guidance body and a part adjacent to it delimit a radial gap through which guidance air can flow radially outwards, the jacket surface of the air guidance body having in its forward section a conoidal surface narrowing conically towards the front and a transitional section connecting the rear radial surface with the conoidal jacket surface. In this embodiment, because the guidance air first flows radially through the gap but then is diverted forwards, a reinforcement of the air stream takes place in that surrounding air is drawn along by the guidance air fed from within. The air flowing forwardly along the jacket surface of the air guidance body is therefore composed of the guidance air supplied and the surrounding air drawn with it. This, too, increases the possibility of influencing the shape of the powder cloud.
It is preferred in this embodiment that the transitional section be composed of a plurality of conical annular faces with differing cone angles. In such an embodiment the guidance air xe2x80x9cadheresxe2x80x9d better to the jacket surface of the air guidance body when diverted from the radial to a substantially axial flow direction than if said surface formed a continuous curve.
It is also advantageous if an annular space communicating with the gap bordering the air guidance body is provided in the housing to conduct the guidance air. Within this annular space an equalisation of pressure can take place, further homogenising the guidance air stream.
An especially favourable possibility of communication between annular space and gap is a screw thread-like groove cut into an axial surface of a part bordering the annular space.
When using the air guidance body according to the invention it can occur that the air flowing along the jacket surface of the air guidance body has a higher pressure at the front end of said air guidance body, where the through-bore emerges, than the powder stream flowing through the through-bore. This can cause unwanted turbulence. To prevent this, according to a further embodiment of the present invention a supporting air arrangement can be provided by which pressurised supporting air can be so fed into the powder stream flowing through the through-bore of the air guidance body that the pressure prevailing at the mouth of the through-bore does not fall below the pressure prevailing at the jacket surface in that area. In this way turbulence which could disturb the formation of the powder cloud can be reliably avoided.